Hey mama, I think the ones that stack and screw tightly are the best? I used to send the ones from Tommee Tippee that actually fit INSIDE the milk bottle so it’s so convenient - fill with water, take out the container and pour in the exact amount measured and mix. I’ve enclosed pics of it here. Wonder if Pupsik Studio still sells them…
This is how they fit in the bottle - so cool! Sorry nerdy mama here LOL